1. Field of the Invention
The present invention relates to a carbon dioxide absorbent, a carbon dioxide separating apparatus and a reformer, and more particularly to a carbon dioxide absorbent and a carbon dioxide separating apparatus for separating and recovering carbon dioxide from an exhaust gas, a raw material gas or a fuel gas generated from an energy plant, a chemical plant, an automobile and the like which use a raw material or a fuel containing hydrocarbons as a major component; and a reformer for producing and recovering hydrogen from the raw material gas containing carbon such as methane.
2. Description of the Related Art
In order to separate and recover the carbon dioxide in an exhaust gas in combustion equipment such as a motor or the like for burning a fuel containing hydrocarbon as a major component, it is effective to separate and recover the carbon dioxide in the vicinity of a combustion chamber where the concentration of carbon dioxide is high. Thus, the temperature at such a carbon-dioxide recovering place is often a high temperature of 300° C. or more.
In addition, a reaction system is known in which hydrogen is produced as a main produced gas, and carbon dioxide is produced as a by-product gas. For instance, a steam reforming reaction is known in which a fossil fuel containing hydrocarbons as a major component is allowed to react in the coexistence of steam and a catalyst to produce hydrogen as the main product and carbon dioxide or carbon monoxide as the by-product. Furthermore, in chemical industrial processes, a reaction is used in which carbon monoxide is allowed to react with water to produce hydrogen as the main product and carbon dioxide as the by-product. Since the hydrogen as the main product obtained in the reforming reaction and the chemical industrial process is used for a fuel or a raw material, it is required to increase a recovery rate of the hydrogen. In a reaction in which carbon dioxide is produced as the by-product as in the reforming reaction or shift reaction, carbon dioxide is removed from the reaction field. For example, JP-A 11-263988 (KOKAI) discloses separating carbon dioxide from the reaction field of steam reforming. As a result of such removal of carbon dioxide from the reaction field, the chemical equilibrium shifts to the production side of the main product. Accordingly, it becomes possible to increase the recovery rate of a gas as the main product. These reforming reaction and shift reaction are carried out at a temperature of 400° C. or more.
As a conventional technology for separating carbon dioxide from a gas, there are a chemical absorption process by means of an alkanol-amine-based solvent or the like, a pressure swing method, a cryogenic distillation method, a membrane separation process and the like. However, it is required to suppress the upper limit of the temperature of an introducing gas to around 200° C. in any of these methods due to the limit of heat resistance of materials such as a membrane and a solvent to be used for separation of carbon dioxide.
In order to effectively separate and recover the carbon dioxide in a gas exhausted from combustion equipment such as a motor, the process must be conducted in an environment of 300° C. or more. In addition, it is required to remove the carbon dioxide at a temperature of 400° C. or more in a reforming reaction or a shift reaction. In this respect, it has been difficult to remove carbon dioxide from the reaction field by a conventional method.
Under the circumstances, JP-A 2000-262890 (KOKAI) discloses a method of separating carbon dioxide from a high-temperature gas containing the carbon dioxide in a temperature range exceeding 500° C. by using a lithium composite oxide which reacts with the carbon dioxide without accompanying a cooling step. In JP-A 2002-274809 (KOKAI), there is disclosed a method in which a reaction vessel for carrying out a reforming reaction or a shift reaction is filled with a lithium-containing oxide such as lithium zirconate, lithium orthosilicate and the like, whereby carbon dioxide is removed from a high-temperature reaction field exceeding a temperature of 400° C. to effectively obtain a principal product. For instance, in the case of a methane steam reforming system using of a reaction vessel filled with lithium orthosilicate together with a methane reforming catalyst, the steam reforming reaction expressed by the following formula (1) occurs concurrently with an absorption reaction of carbon dioxide by means of lithium orthosilicate expressed by the formula (2) in a reaction vessel of a temperature of around 400 to 650° C.CH4+2H2O4H2+CO2  (1)Li4SiO4+CO2Li2CO3+Li2SiO3  (2)
The absorption reaction of carbon dioxide (the reaction directing to the right side) by means of lithium orthosilicate which is expressed by the formula (2) proceeds at the fastest at a temperature of around 600° C. A temperature range of the carbon dioxide absorption reaction varies depending on the carbon dioxide concentration under the reaction environment, so that the upper limit temperature of the absorption reaction temperature range increases with the increase in carbon dioxide concentration. When lithium orthosilicate is used to remove carbon dioxide from the steam reaction field of methane, the reaction equilibrium expressed by the formula (1) shifts to the reaction of hydrogen production directing to the right side, whereby the reforming reaction of methane is promoted, resulting in improvements in the recovery percentage of hydrogen. When the lithium orthosilicate which has absorbed carbon dioxide is heated, the reaction expressed by the formula (2) directing to the left side occurs to release the carbon dioxide. Accordingly, the lithium orthosilicate is regenerable. As described above, when a reaction vessel in which a reforming reaction or a shift reaction is carried out is filled with lithium orthosilicate, it becomes possible to remove carbon dioxide and to efficiently and repeatedly obtain hydrogen as the main product.
In this respect, however, when only lithium orthosilicate is used, the absorption rate of carbon dioxide is low so that sufficient speed cannot be obtained. Hence, in JP-A 2001-96122 (KOKAI), it is disclosed that an alkali carbonate such as potassium carbonate, and sodium carbonate is added to lithium orthosilicate, whereby the absorption rate of carbon dioxide is improved to increase the absorption performance.
It has been found, however, that when low concentration carbon dioxide is absorbed by using lithium orthosilicate or lithium orthosilicate to which lithium alkali carbonate is added, a sufficient carbon dioxide absorption rate cannot be obtained. For this reason, when such lithium orthosilicate is used as a carbon dioxide absorbent for absorbing carbon dioxide in a gas used for a methane steam reforming reaction, a shift reaction or the like, a methane conversion rate is low so that hydrogen gas having a high concentration cannot be obtained.